1. Field of Invention
This invention relates to a method and apparatus for monitoring the integrity and timing of a fluid dispense. More particularly, the present invention relates to a method and apparatus for monitoring the integrity of a fluid dispense in real time.
2. Description of the Prior Art
A variety of industries require the delivery of precise volumes of a liquid at a consistent rate for highly repeatable periods of time. Furthermore, at the end of a delivery, subsequent processing will occur whose timing with respect to the delivery is crucial for process consistency and product uniformity. For example, precise volumes of a photoresist composition are delivered to a silicon wafer substrate in order to form a photoresist layer of uniform thickness onto the wafer. The wafer is spun at high speeds following the liquid delivery to evenly distribute the liquid on the wafer. The requirement for precise volumes, duration of dispense, and rate of delivery must be affected by an apparatus which is delivering the same volumes of liquid repeatedly at a consistent delivery rate for a highly repeatable duration over long periods of time.
As the semiconductor industry expects to utilize submicron technology, photo resists and low dielectric liquid compositions are becoming increasingly important for improving the performance of microprocessor and memory storage devices. To reduce manufacturing costs and reduce photoresist waste, lower dispense amounts of liquid photoresist and lower viscosity of liquid will be dispensed. Coatings on the wafer of these fluids require a high degree of uniformity in the thickness across the wafer and also the thickness from wafer to wafer. In order to attain the required uniformity and high manufacturing yield of films formed from these fluids, the fluid dynamics and timing of the liquid dispensed onto the wafer are critical. For example, broken streams of liquid impinging on the wafer, caused by drops or air bubbles in the dispense nozzle or tubing, are common causes of wafer defects which effect the uniformity of the film formed from the liquid. Increase defects lead to undesirable increases in production costs.
Fluids dispensed onto a wafer are made into a uniform film on the wafer by spinning the wafer at low speeds as the fluid is dispensed and then increasing the spinning speed, a step called spin up, to a final spin speed to uniformly spread the liquid across the wafer and to cause evaporation of the photoresist or low K dielectric solvent. The time during which the wafer is at its final spin speed is crucial to forming reproducibly thick and uniform coatings. Knowing the time between the end of a fluid dispense, when the liquid leaves the nozzle, and the time to final spin up is important for controlling the time the wafer is spun at the high spin speed and thus effects coating uniformity. The importance of various variables on coating uniformity is described by Daughton and Givens in J. Electrochern. Soc., (1992), vol. 129, p 173.
It has been proposed in PCT application number 98/00736 to provide and electronic rain gauge. A container is provided for collecting rain water and converting the collected water into drops of defined volume through an orifice. The volume of rain is determined by counting the drops of liquid from the collector. This is accomplished by an optical emitter and optical receiver pair such that when the liquid drop passes between the emitter and receiver a voltage change or pulse is produced. By counting the number of voltage pulses and multiplying by the volume of a drop, the total amount of precipitation can be determined. No means is provided for monitoring the shape of the drop, its time leaving the apparatus, nor are means provided for detecting irregularities, the shape of the drop, or integrating the output signal from the emitter sensor pair as an input signal into a wafer spin coater or to the fluid dispense pump. In contrast, when monitoring a liquid dispense onto a silicon wafer, it is necessary to provide a means for monitoring the dispense in a manner that is capable of determining that the dispense is applied to each wafer in a consistent manner including the time of the dispense, shape of the dispense fluid, as well as determining the dispense volume. This requirement renders it necessary to monitor the shape and timing of the fluid dispense so that the rate of fluid application to the substrate from initial contact, intermediary contact and final contact between the fluid and substrates can be essentially duplicated over an extended period of time.
Accordingly, it would be desirable to provide a method and apparatus for monitoring the delivery of precise volumes of liquid with repeatable rates, fluid dynamics, volumes, and intervals of delivery. In addition, it would be desirable to provide such a method and apparatus which is capable of monitoring the delivery of precise volumes of liquid at repeatable rates over extended times, in a production line on a real time basis and using that information for process control.
In accordance with this invention, a time dependent profile of the dynamics of a liquid being dispensed from a nozzle is generated and analyzed. The profile thus generated then is compared to a standard profile that correlates in time, volume, and profile to a satisfactory liquid dispense. When the profile which is generated is deemed to be satisfactory in volume, timing, and shape, further processing of the liquid dispensed onto a substrate such as a silicon wafer is continued to effect further treatment of the liquid coated substrate. A light generator and a light detector are mounted below the nozzle from which the liquid is dispensed and the light generator and light detector are positioned on opposite sides of the path of liquid dispense flow. It is to be understood that other forms of energy from a generator, including but not limited to thermal, acoustic, or other types of electromagnetic energy can also be used in conjunction with a suitable detector. Thermal energy emitted from the sample itself can also be considered as a source of energy for a suitable detector. While light energy will be used as an example for description of this invention it is understood that these other types of energy and sensors could be used interchangeably.
During the dispensing process, the liquid stream will absorb or scatter some of the light from the light generator while the light transmitted from or scattered through the liquid is collected by the detector. The amount of light transmitted from the stream, whetther it be absorbed or scattered, depends upon the diameter and the continuity of a given liquid stream emanating from the fluid dispense nozzle. As the rate the liquid is dispensed changes or stops, the diameter of the liquid stream reduces and breaks and the amount of light that reaches the detector is increased. By recording the amount of light collected over the time and converting that measurement to a suitable electrical signal, the time dependent profile, the fluid dynamics, of the dispense is obtained. This signal is then compared to a previously generated standard signal for a satisfactory liquid dispense to determine the acceptability of the dispense for its intended purpose. Alternatively, the signal obtained from the sensor can be used to define the timing of the dispense and end of dispense, and be used to start subsequent fluid processing steps such as spin-up or the application of a reduced pressure to the chamber.
In accordance with this invention, a process and system are provided for monitoring the light or energy transmission characteristics of a fluid dispense and correlating the monitored characteristics with a previously generated standard dispense to determine whether the fluid dispense is satisfactory for a particular purpose such as for a coating on a substrate. Also provided by this system is a means to determine the time at which the last fluid has left the dispense nozzle and to use the signal provided to initiate subsequent processing steps. A fluid is dispensed from a nozzle and is passed through at least one detector set, each comprising an energy emitter and an energy detector. As an example, the photons from an emitter pass through the fluid dispense and the photons transmitted through the fluid dispense are detected by the detector. A graph correlating degree of photon transmission over time then is compared to the standard graph to determine whether the dispense is satisfactory. The entire volume of dispense is exposed to the emitted photons so that the graph characterizes the entire dispense. When the dispense is deemed satisfactory, the coating process or the like in which the dispense is used, such as spin-up, is allowed to continue. When the dispense is deemed unsatisfactory, the object of the dispense is removed from further processing. For example, when coating a silicon wafer with a photoresist or low K dielectric or other material, the wafer coated with an unsatisfactory dispense is removed from further processing and the dispense fluid is removed there from such as by solvent extraction. In this example, substantial economic savings are realized by permitting recovery of acceptable untreated wafers that would become unacceptable if processed further such as when unsatisfactory photo-processing of the coated wafer is effected.
An alternative embodiment of this invention would measure the energy reflected or scattered from the substrate through at least one detector set, each comprising an energy emitter and an energy detector. The energy from the emitter strikes the fluid dispensed onto the substrate and the energy reflected from the fluid dispensed on the substrate is detected by the detector. A graph correlating degree of energy scattering over time then is compared to the standard graph to determine whether the dispense is satisfactory. The wafer is exposed to the emitted energy so that the graph of the scattered or reflected energy characterizes the entire dispense onto the wafer.